Treatment of aluminum oxide coatings



llnited States Patent U M delphia, Pa.

No Drawing. Filed Oct. 5, 1961, Sier. No. 143,034

5 Claims. (Cl. 204-35} This application is in part a continuation of my application Serial No. 92,156, filed February 28, 1961.

This invention relates to the treatment of aluminum oxide coatings of porous type artificially formed on aluminum or aluminum-base alloys to produce increased resistance to corrosion, the treatment involving specifically the use of alkali metal silicates. Hereafter, for convenience, aluminum and aluminum-base alloys will be collectively designated as aluminum.

Porous coatings to which the treatment described herein applies are such as are produced on aluminum by anodizing or by chemical treatment in particular Ways. The coatings, as produced by anodizing, involve anodizing in such electrolytes as are typified by sulfuric acid (or equivalent bisulfate), phosphoric acid, chromic acid, sulphamic acid, oxalic acid, or other electrolytes producing similar results. The oxide coatings thus produced are porous, in contrast with those produced by anodic treatment in boric acid, ammonium bicarbonate, or similiar electrolytes which produce coatings of such low porosity that the matters to be herein described are inapplicable.

Other porous coatings to which the present invention is applicable are producible by chemical oxidation and are typified by those produced by the use of alkali chromates combined with sodium carbonate, or by the use of baths containing fluorine ions and chromic ions, in in some cases with the addition of phosphoric ions, e.g., an aqueous acid bath comprising chromic acid, sodium bifiuoride, sodium fluoborate and a sodium phosphate.

The production of porous oxide coatings, both electrolytically and by non-electrolytic chemical action, is well-known, and it is unnecessary to describe the details of the processes used which are widely applied in this art. The present invention is quite generally applicable to the porous coatings so produced.

Alkali metal silicates have been used in the treatment of porous aluminum oxide coatings in attempts to protect such coatings against corrosion. However, the corrosion resistance developed by known methods has not been satisfactory because they merely deposit loose, sparingly absorbable protective films most of which may be easily wiped from the coatings. A coating protected by such a film may, directly after formation, stand up under the usual corrosion resistance tests. But when the coating is first wiped and then tested, the coating from which the film is more or less removed will not stand up satisfactorily. Further, in the use of known methods of us ing silicates the relatively loose protective film becomes chalky when exposed to air, and the surface readily becomes streaked.

The general object of the present invention may be briefly stated as that of producing greatly improved corrosion resistance through the use of alkali metal silicates but with the silicates involved in a fashion which is, apparently, one not involving a mere coating of the aluminum oxide coating. This is indicated by the fact that even when thoroughly wiped down the coating not only has exceptional corrosion resistance but also retains its initial uniform appearance.

The invention is characterized by the use of successive steps. In the first step of the treatment here involved (which may be preceded by other steps not related di- Zi,l7d,9lfi Patented Mar. 23, 1965 rectly to the invention) a preliminary treatment of the porous oxide coating is effected by the use of a silicate solution under conditions which fall short of producing sealing action. Either sodium or potassium silicate may be used, but since sodium silicate is considerably less costly it constitutes the commercial chemical and reference will be specifically made hereafter to the use of sodium silicate, with the understanding that equivalent quantities of potassium silicate may be used. Preferably the first treatment is carried out at a temperature less than 120 P. if successive treatments are carried out in different silicate baths) and best results have been secured when the temperature used is around room temperature up to about F. The time of this treatment should be relatively short: /2 minute to 5 minutes. For the first treatment the ratio of silica to alkali metal oxide should be at least 2:1. The concentration of the silicate may range from about 1 gram per liter to saturation, this range involving equivalent concentrations of silicate ranging from about 0.1% to 30%. The maximum concentrations used will depend upon the solubility of the particular silicates involved, high concentrations being undesirable when the viscosity of the silicate solution becomes unduly high causing it to adhere to the work being treated.

Times of treatment are not critical, but in this first step they should desirably be less than about five minutes, two minute treatment being generally sufiicient, longer times of treatment being undesirable particularly when the silica to soda ratio is low, giving alkalinity conditions which may cause objectionable etching.

The action which occurs in this first step is open to conjecture, but it seems probable that aluminum silicate is formed producing a reduction of pore size. It is not desirable to carry out this step at temperatures above F. (still referring to successive treatments in different silicate baths) because then there may be hydra tion impeding the desirable action of the first step. Whatever occurs in this first step should become complete or effective before any substantial degree of hydration may occur.

But alternative to what has just been described is a first treatment in the same silicate bath as that used in the second step and at an elevated temperature. In this case advantage is taken of the fact that sealing with silicate is a function of both temperature and time and sealing is prevented if, even though the coated aluminum articles are immersed in a hot silicate solution, they are quickly removed therefrom and held in the air for a short time before being again immersed in the silicate solution for the final treatment. The explanation is probably that, because the articles cool off very rapidly upon removal from the bath, the same action takes place as occurs if the initial treatment took place at a considerably lower temperature.

For example, considering a bath of sodium silicate having a ratio of silica to sodium oxide of 2:1, with a silicate concentration of 5 grams per liter, and maintained at 210 B, an immersion of the coated aluminum articles for 1 minute or less followed by removal into the air for /2 minute to 1 minute or more before a second immersion in the same bath, gives results substantially the same as those previously discussed. Apparently this is due to the fact that the first immersion effectively only wets the articles and the first step is primarily carried out after the removal at an average temperature so decreased that the scaling which is to be avoided does not occur.

In commercial practice the time of the first immersion usually has a lower limit of about /2 minute because the articles carried by racks must be handled by hoists, the

most rapid operations of which in effecting dipping and removal take times of the order of /2 minute; in other useful etlect.

words, while dipping for less time would theoretically suffice, it cannot practically be carried out substantially faster.

The advantage of using the same bath for both the first and second treatments is, obviously, economy and facilitation of operation. Preferably, to avoid substantial sealing in the first treatment the single bath in such case is operated at a temperature relatively low in the range for good results in the second treatment, e.g. as low as 150 F., thereby giving more leeway in the permissible time of immersion in the first treatment. For example, if the bath is at 150 F., the first immersion may take as long as about minutes as compared with little more than 1 minute if the bath is at 210 F. As will more fully appear hereafter, the second treatment must be correspondingly prolonged if the bath temperature is thus maintained low.

It may seem, on casual consideration, that a double immersion in the same bath would only be equivalent to continuous immersion in the bath; but this is definitely not the case. Apparently whatever reaction occurs as a result of the first immersion takes some time (though short) and for proper results must be completed by withdrawal of the articles into the air, with resulting cooling, before they are subjected to substantial sealing action of the silicate. Attempts to secure the desired results in a single immersion have been completely unsuccessful.

In accordance with the present invention, the first treatment, carried out in one of the above fashions just indicated, is followed by a second treatment involving the use of an alkali metal silicate in hot aqueous solution. Either sodium or potassium silicate may be used, but from the practical standpoint sodium silicate is least expensive and is generally used. This second step of the treatment involves the subjection of the coating, following the application of the first step, to the action of a solution of alkali silicate which has a concentration of silicate in the range from one gram per liter to saturation, the latter depending upon the particular silicate which may be used. The concentration of silicate may range from about 0.1% to 30%. Generally speaking, the concentration of silicate in its solution is conveniently about ten grams per liter, though this concentration is far from critical. As to the silicates used, they may involve a ratio of silica to alkali metal oxide in the range of about 1:1 to about 4:1. The temperature of the treatment is desirably above 150 F. and may range up to the boiling point. Temperatures for this second step are preferably in the range from 190 F.

to the boiling point of the solution. The time of treatment of the articles may range from 5 to 30 minutes. If the silica to soda ratio is low, temperatures of treatment in the lower portion of the temperature range should be used to minimize etching action. More extended treatment than 30 minutes is harmless but also has very little Practical considerations indicate that treatment ranging from five to twenty minutes is very satisfactory. While the last discussion is generally worded to cover procedures in which the first and second treatments are carried out in difierent baths, it will be understood from the earlier discussion that the bath for the second treatment may be that used in the first treatment, with the indicated precautions being taken to prevent substantial sealing in the first treatment and to effect completion thereof, while the second treatment is carried out to complete sealing.

In the case of the specific examples referred to below it may be assumed that the aluminum articles are anodically coated in a (by weight) sulfuric acid solution in water with a current density of twelve amperes per square foot of alternating current for from 5 to 60 minutes at 75 F., in accordance with conventional practices. The articles are then washed with water and given the treatments in accordance with the invention. While specific reference has been made to one anodizing procedure, this is merely for illustrative purposes and has no basic sig nificance, any oxide-forming coating process being usable which provides a porous aluminum oxide film, as already described. The choice of one oxide coating procedure over another is generally dictated only by particular specific results desired such as the thickness or density of the coating.

Specific examples of procedures in accordance with the invention are the following:

Example I Anodically porous-coated aluminum articles are immersed for 2 minutes in an aqueous solution at F. containing 5 grams of sodium silicate per liter, the silica (SiO to soda (Na O) ratio being 2:1. The articles are then sealed (no rinsing being necessary) by being immersed for 5 minutes in an aqueous solution at 200 F. conatining 10 grams of sodium silicate per liter, the silica to soda ratio being 2: 1.

Example II Anodically porous-coated aluminum articles are immersed for 2 minutes in an aqueous solution at 120 F. containing 10 grams of sodium silicate per liter, the silica to soda ratio being 3.211. The articles are then sealed by being immersed for 5 minutes in an aqueous solution at 200 F. containing 10 grams ofsodium silicate per liter, the silica to soda ratio being 3.221. For this sealing the ratio of silica to soda may be anywhere between 3.221 and 1:1, or more or less.

Example 111 Anodically porous-coated aluminum articles are immersed for 5 minutes in an aqueous solution at room temperature containing 10 grams of sodium silicate per liter, the silica to soda ratio being 2:1. The articles are then sealed (no rinsing being necessary) by being immersed for 5 minutes in an aqueous solution at F. containing 10 grams of sodium silicate per liter, the silica to soda ratio being 2:1.

The foregoing examples involve the use of separate baths for the initial and final treatments. Examples may now be given which involve successive immersions of the articles in the same bath.

Example IV Anodically porous-coated aluminum articles are immersed for 1 minute in an aqueous solution at 210 F.

containing 5 grams of sodium silicate per liter, the silica to soda ratio being 2:1. The articles are then removed and maintained in the air above the bath for 1 minute, after which they are again immersed in the same bath for 5 minutes.

Example V The same procedure as in Example IV is carried out except that the times of the first immersion and the maintenance of the work in the air were reduced to the .minimum practical commercial times of /2 minute, each.

The results were identical with those of Example IV.

In a laboratory test the last procedure was carried out but with the time during which an article was held in the air between treatments reduced to 10 seconds. The results were still identical with those of Example IV.

The last two examples were carried out using aluminum articles containing porous coating produced, respectively, (1) by convention treatment with an aqueous solution of 6 ounces per gallon of sodium carbonate and 2 ounces per gallon of sodium chromate, and (2) by conventional treatment with an aqueous solution containing, per liter, 3.7 grams of chromic acid (CrO 0.75 gram of sodium bifluoride (NaHF 0.7 gram of sodium fluoborate (NaBF and 1.0 gram of trisodium phosphate for 5 minutes at 80 F. The results produced were the same as in Example IV, above.

The sealed oxide coatings produced in accordance with the foregoing examples will stand up for at least sixv ha 6.? teen hours when subjected to a salt-spray test carried out with an aqueous solution containing sodium chloride and one gram per gallon of cupric chloride, adjusted to a pH of 3.2 with acetic acid, the solution being maintained at a temperature of 120 F. Furthermore, .sealed oxide coatings so produced will stand upfor from eight to ten minutes when immersed in an aqueous solution containing 3% caustic soda and applied at 80 F. Such tests respectively indicate high resistance to salt spray corrosion and also to alkali corrosion.

With respect to the matter of both salt-spray and alkali-corrosion resistance, it may be noted that there has arisen a considerable demand in certain industries for such combined resistance. In the automobile industry, for example, washing compounds for cleaning cars are sometimes very high in alkalinity, in consequence of which they have a tendency to corrode anodized aluminum parts. In the architectural field, the corrosive actions of cements and mortars are often highly alkaline in nature, tending to corrode anodized aluminum.

As already indicated, a sodium silicate solution may be used for the initial treatment which is identical, as to concentration and silica to soda ratio, with the sodium silicate of the second sealing treatment, whether the same bath is used or different baths maintained at different temperatures. When sodium silicate is used for both the initial treatment and the second sealing treatment and the solution of the latter treatment is more highly concentrated than that of the initial treatment, a water rinse may be used between the first and sealing treatments. Further, if the sodium silicates of the two treatments do not have the same silica to soda ratio, a Water rinse may be used between the first and sealing treatments. This rinsing merely prevents change of composition of the second bath by carry-over from the first.

What is claimed is:

1. The method of treating aluminum having a porous oxide coating thereon which comprises first applying to the surface thereof a hot aqueous solution of an alkali silicate for a time preventing substantial sealing of the oxide coating, exposing the thus treated aluminum to the cooling effect of air to complete the resulting reaction, and then applying to the surface thereof an aqueous solution of an alkali silicate at a temperature in excess of about 150 F. for a period ranging from about 5 minutes to 30 minutes to complete sealing thereof, said successive treatments being with the same aqueous solution of alkali silicate.

2. The method of treating aluminum having a porous oxide coating thereon which comprises first applying to the surface thereof an aqueous solution of an alkali silicate at a temperature not in excess of about F. for a period ranging from about /2 minute to 5 minutes, and then applying to the surface thereof an aqueous solution of an alkali silicate at a temperature in excess of about F. for a period ranging from about 5 min utes to 30 minutes.

3. The method of treating aluminum having a porous oxide coating thereon which comprises first applying to the surface thereof an aqueous solution of an alkali silicate, having a silica to alkali metal oxide ratio of at least 2:1, at a temperature not in excess of about 120 F. for a period ranging from about /2 minute to 5 minutes, and then applying to the surface thereof an aqueous solution of an alkali silicate at a temperature in excess of about 150 F. for a period ranging from about 5 minutes to 30 minutes.

4. The method of treating aluminum having a porous oxide coating thereon which comprises first applying to the surface thereof an aqueous solution of an alkali silicate at a temperature not in excess of about 120 F. for a period ranging from about /2 minute to 5 minutes, and then applying to the surface thereof an aqueous solution of an alkali silicate, having a silica to alkali metal oxide ratio of 1:1 to about 4:1, at a temperature in excess of about 150 F. for a period ranging from about 5 minutes to 30 minutes.

5. The method of treating aluminum having a porous oxide coating thereon which comprises first applying to the surface thereof an aqueous solution of an alkali silicate, having a silica to alkali metal oxide ratio of at least 2:1, at a temperature not in excess of about 120 F. for a period ranging from about /2 minute to 5 minutes, and then applying to the surface thereof an aqueous solution of an alkali silicate, having a silica to alkali metal oxide ratio of 1:1 about 4: 1, at a temperature in excess of about 150 F. for a period ranging from about 5 minutes to 30 minutes.

References Cited by the Examiner UNITED STATES PATENTS 2/34 Edwards 20435.1 7/35 Tosterud 20435.1 

1. THE METHOD OF TREATING ALUMINUM HAVING A POROUS OXIDE COATING THEREON WHICH COMPRISES FIRST APPLYING TO THE SURFACE THEREOF A HOT AQUEOUS SOLUTION OF AN ALKALI SILICATE FOR A TIME PREVENTING SUBSTANTIAL SEALING OF THE OXIDE COATING, EXPOSING THE THUS TREATED ALUMINUM TO THE COOLING EFFECT OF AIR TO COMPLETE THE RESULTING REACTION, AND THEN APPLYING TO THE SURFACE THEREOF AN AQUEOUS SOLUTION OF AN ALKALI SILICATE AT A TEMPERATURE IN EXCESS OF ABOUT 150*F. FOR A PERIOD RANGING FROM ABOUT 5 MINUTES TO 30 MINUTES TO COMPLETE SEALING THEREOF, SAID SUCCESSIVE TREATMENTS BEING WITH THE SAME AQUEOUS SOLUTION OF ALKALI SILICATE. 